Molar mass of NaCl = 58.44 g/mol
Number of moles :
n = mass / molar mass
n = 21.04 / 58.44
n = 0.36002 moles
Volume = 750 mL in liters 750 / 1000 = 0.75 L
Molarity :
M = number of moles / Volume in liters
M = 0.36002 / 0.75
M ≈ 0,4800 mol/L⁻¹
hope this helps!
